1. Field of the Invention
The present invention relates to a rotationally extendable structure (rotational telescoping structure/rotationally retractable structure) of a rotational ring that is a constituent member of a lens barrel.
2. Description of Related Art
In regard to a rotational ring which is provided in a lens barrel and moves in an optical axis direction while rotating about the optical axis, a structure is employed in which a rotational driving force of a motor is transferred to the rotational ring via a peripheral gear, provided on the outer periphery of the rotational ring, and a gear (rotational-ring drive gear) that is in mesh therewith. Such a structure is disclosed in, for example, Japanese Unexamined Patent Publication Nos. H11-52211 and 2002-168308.
Generally, the above-described rotational-ring drive gear is supported by a stationary member, and the length of the rotational-ring drive gear needs to be long enough in order to maintain engagement with the peripheral gear of the rotational ring throughout the entire movement range of the rotational ring in the optical axis direction. On the other hand, it is necessary to provide a reduction gear train (gear reduction mechanism) between the motor and the rotational ring; however, due to the rotational-ring drive gear being configured as a long gear that can mesh with the peripheral gear of the rotational ring throughout the entire axial length of the rotational-ring drive gear, the gear (of the reduction gear train) that is directly in mesh with the rotational-ring drive gear must be configured as an idle gear, having no gear-reduction capability, in order to avoid interference with the rotational-ring drive gear (long gear); hence, a large number of gears are required for the reduction gear train, which becomes a cause of restricting the downsizing (miniaturization) of the lens barrel. Whereas, if the rotational-ring drive gear itself is configured as a double gear having a gear-reduction capability in order to increase the reduction gear ratio, a large-diameter reduction gear, which is coaxial with the long gear, needs to be added to (formed on) the long gear that is in mesh with the peripheral gear of the rotational ring. Hence, restrictions are brought on the movement of the rotational ring due to the presence of this large-diameter reduction gear, which is also a cause of restricting the downsizing (miniaturization) of the lens barrel.